As motor drive devices of a type having a function of limiting overcurrent, there has been proposed in the past a motor drive device that monitors a value of electric current to a motor and restricts driving operation of the motor when a value of the detected current exceeds a threshold value (refer to Patent Literature 1 for example). Such type of the motor drive device has a PWM (Pulse Width Modulation) drive circuit, and it is configured to limit a duty factor of PWM signals to a predetermined ratio according to a detected value of the motor current when the detected value of the motor current exceeded the threshold value, that is, when an overcurrent is considered to be flowing to the motor. The conventional motor drive device of such a configuration prevents the motor from being overheated and burn-damaged due to the overcurrent by means of suppressing abrupt changes in the motor current.
However, the conventional motor drive device discussed above causes the duty factor to change instantaneously and greatly if a value of the motor current is large at the time of determination of the overcurrent since it limits the duty factor to a ratio corresponding to the value of motor current when determined that there is an overcurrent. Such an instantaneous and great change in the duty factor influences substantially upon rotation of the motor, and it thus gives rise to a problem that rotating sound of the motor increases. There is a case where the motor drive device repeats switching between a mode of small duty factor affected by the limiting function due to the overcurrent and another mode of large duty factor after relieved from the limiting function especially when the motor carries a heavy load or the like instances, and repetition of this nature causes such troubles as motor noise and large impacts to mechanical components.    PTL1: Japanese Patent Laid-Open Publication No. 2005-199899